Masco ME-8 - no sound...

[bluzmn]

I finally got a chance to work on the "for parts or repair" Masco ME-8 I got on eBay. I installed a grounded cord and started bringing it up slowly on a variac (10 volts every 10 minutes or so). At around 40 volts the on/off light started to glimmer; at around 50 or 60 volts the tubes started to show a little glow. At around 70 or 80 volts, I heard aloud pop, then a sizzle; I shut it down and replaced the filter caps. I brought it up on the variac again; at 120 volts the tubes and the on/off light were glowing bright, but I got no sound. Could a bad cathode cap on the 6SJ7 be causing that, or is there something else I should be checking?
By the way, once I examined the layout it became clear that C4, which some of you thought was drawn upside down on the schematic by mistake, is part of the tone circuit; it has nothing to do with the cathode of the 6L6.

[Enzo]

If you are so afraid of what might happen that you are bringing it up on a variac that way, why are tubes even in it?

I don;t mean that flippantly.

Bring it up without tubes just to see if the transformer and heater wiring and stuff are OK. Bonus, no filter caps will see voltage that way - at least they won;t unless some incredible short circuit has occurred somewhere. Verify heater voltage at the sockets - including the 5v. Verify HV at the rectifier socket.

Then put in just the rectifier tube, and bring it up slowly. Now we are testing the rectifier itself of course, but we are also checking the B+ filters. And at the same time we are also checking the output transformer to see if its primary is shorted to core or secondary.

IN any amp, one of the first checks to make is power supply. I weouldn;t be even thinking about cathode caps until I had verified power supply. Do you have B+ at ALL the B+ nodes? Is there B+ voltage at every plate pin on each empty socket? Remember we have the rectifier in already. If we have that, THEN we install the rest of the tubes and see what happens.

The first B+ node, which only goes to the OT primary is 300v, and the second node (serving everything else) is 280v. Without tubes, all those plate pins ought to read about 300v. With tubes in, note the plate voltages on the first two fall to 100v and 80v. Your voltages may be quite different from that, but if your B+ is somewhere in the 300v range, then do your plate voltages fall to somnething in the 100v range with tubes? If so, that means at least the tubes are conducting. If either stays high, then that tube is not conducting.

The first tube - for the mic input - has a grounded cathode and a 15 Meg grid resistor, classic grid leak bias.

The second tube, the pentode has conventional cathode bias. The schematic says 1.3v upper left side of the tube at the suppressor grid. That is wired to the cathode, which would ALSO be at 1.3v. The schematic says 80v next to the cathode, which is clearly wrong. The plate says 80v. That would mean the tube has a normal zero volts across it - a dead short. Not gonna happen. Look for 1.3v there. how about something between 1 and 2 volts as close enough. That is also a sign of tube conducting.

And your 6L6 shows a -7.5v bias at the grid. You get 5 to 10v negative there?

If all that is OK< it should amplify. If it doesn;t, we troubleshoot.

You have no sound? Does that mean no signal comes through? Or is it UTTERLY silent? Is there any little background hum or hiss, or does it sound the same as if it were turned off? The difference points us different ways.

If the voltages are present, touch the grid pin 5 on the 6L6 with something - a small screwdriver or maybe your meter probe but without grounding the other probe. That should inject some hum into the thing. Get hum out the speaker? If so, the power amp is working, if not, then it is the problem

Assuming that works, go back to the pentode. TOuch the grid - looks like pin 4 if the drawing is right. Get hum from that? If so, it works, if not, then the trouble is between that grid and ther 6L6 one.

Now go to the phono input - put a signal to it. If the pentode stage workerd and this doesn;t, suspect the volume control or open R7.

Get that working before worrying with the4 mic. Once those work, then try the mic input, max that volume control. Touch that grid pin, hum? No? Touch the wiper of the control, then the top lead, then eithe side fo the cap. What we want to do is move back until we find the point it can't pass.

6SJ7 cathode cap? Of all the parts in this, how did you decide on that one? Just wondering. No, if that cap shorts, it shouldn;t kill the signal, just make it sound bad. The caps I might worry about are C7 and C9 - coupling caps from plates to grids. Those get leaky and they let DC onto those grids. And caps this old, well...

You need to troubleshoot systematically to find WHERE the problem is, then we can determine WHY it occurred. For example it is at some point X in the circuit, and the why is an open resistor.

Apparently this amp has been discussed elsewhere, I didnl;t see that. But C4 has nothing to do with the cathode of the 6L6, because looking at the schematic, it isn't connected to the cathode. It is however the filter cap for your bias supply to the grid - that -7.5v - and it sure looks upside down to me if it is filtering -7v. It has nothing to do with a tone circuit, other than the tone cap C10 is wired to the same grid.

[bluzmn]

Enzo, thank you for that concise description of the "bringing it up on the variac" process. Why was I bringing it up on the variac with tubes in it? Because I have never found, in a book or on a forum, such a systematic description of the process. Some just say "bring it up slowly on a variac"; some do say to take out the tubes, but not when and in what order to put them in. So I've been doing it with the tubes in; I figured it was better than slamming the amp with 120v all at once. And I have caught some faulty components before they became catastrophic.
By "no sound" I meant that I plugged in a mic and I got no sound; there was a slight background hum.
I asked about the 6SJ7 cathode cap because electrolytic caps that are that old are known to go bad.
And actually both of those capacitors are grounded to pin 8 of the 6L6; their other end is connected to the tone control, which is connected to the grid. Neither of those caps appears to be attached directly to pin 5.
Here's another schematic I've found:

[Enzo]

C4 is not connected to the grid directly, it is a filter cap, it is on the other end of the tone pot, yes, but the bias flows through that pot to get to the grid.

You are correct, neither cap is wired to pin 5, but they are still serving that grid from where they sit. C4 smoothes the bias supply, and the other cap is the tone cap.


Oh it isn;t like you did something wrong, just there is a purpose to the procedure, so might as well address the needs of that purpose directly.


Did my step by step journey through the signal path make sense to you? If you get background noise, it is likely the 6L6 stage is working, at least well enough to make sound.

[bluzmn]

Yes, it makes perfect sense; thanks again for the guidance.